1. Field of the Invention
The present invention relates to devices for forming images of an examination subject utilizing nuclear magnetic resonance tomography, and in particular such an apparatus having adjustable capacitors wherein the motors for moving the capacitor plates are operated by the basic magnetic field of the apparatus.
2. Description of the Prior Art
Tomography devices are known which operate based on the concept of nuclear magnetic resonance, having means for applying a basic magnetic field as well as a gradient field to the examination subject, and for detecting deflection of the atomic nuclei of the examination subject from their normal state of equilibrium by the presence of a high frequency magnetic stimulation pulse. Such a device is described, for example, in German OS 31 24 435, corresponding to U.S. application Ser. No. 379,998 (Loeffler et al) now abandoned assigned to the assignee of the present subject matter.
Such devices operate by deflecting atomic nuclei, such as the hydrogen nuclei, of an examination subject from a preferred orientation, the preferred orientation being generated by a basic magnetic field, and the deflection occurring as the result of a high frequency stimulation pulse. After the end of the stimulation pulse, the atomic nuclei, due to their spin, return to the preferred orientation after a certain delay. During the delay, the atomic nuclei precess with a frequency which is dependent upon the intensity of the basic magnetic field. If a field gradient is superimposed on this uniform basic magnetic field, so that the magnetic field distribution varies spatially, identification of selected positions within the examination subject is accomplished by measuring the frequency. By selectively altering the direction of the field gradient, layered or tomographic images of the examination subject can be obtained. Stimulation of a selected layer of the examination subject is achieved by applying the field gradient with respect to the basic magnetic field such that excitation of atomic nuclei in the examination subject occurs only in the layer of interest. This is possible because such excitation occurs only with a frequency which is precisely associated with the magnetic field in the desired layer.
A problem in the construction and operation of such NMR tomographic devices is the necessity of precisely adjusting the resonance of the measurement and stimulation coil. For this purpose, the particular coil can be connected with a capacitor arrangement, the capacitor arrangement being tuned corresponding to the desired frequency. Variable capacitors may be employed for this purpose. The coil to be tuned in this manner is disposed in the interior of the basic magnetic field. For adjusting the plates of variable capacitors, electro-motors must also be disposed in the interior of the basic magnetic field. Conventional motors with iron cores are not suited for this purpose because such motors are not operative in the relatively strong magnetic field generated by the basic field coil.